1. Field of the Invention
The present invention relates to a recording apparatus for recording an image on a recording medium by recording means, and more particularly to a recording apparatus having a transport mechanism for transporting a recording medium.
2. Related Background Art
A recording apparatus having a function of a printer, a copier or a facsimile device or which is used as an output device of a complex machine such as a computer and a word processor, or a work station is constructed to record an image on a recording medium such as a paper sheet or a plastic sheet (OHP sheet) in accordance with image information. The recording apparatus is categorized by its recording method of the recording means thereof into ink jet type, wire dot type, thermal type, thermal transfer type and laser beam type.
In a serial type recording apparatus which adopts a recording method of main scanning orthogonally to the direction of transport of the recording medium (sub-scan direction), the recording medium is set at a predetermined record position and then an image is recorded (main scan) by the recording means (a recording head) mounted on a carriage which moves along the recording medium, and after one line of recording, the recording medium is fed (sub-scan) by a predetermined amount and the image of the next line is recorded (main-scan). This process is repeated to record the entire image on the recording medium. On the other hand, in a line type recording apparatus which records an image by only the sub-scan, that is, the feeding of the recording medium along the transport direction, the recording medium is set at a predetermined record position, one line of image is recorded at one time, the recording medium is fed by a predetermined amount (pitch feed) and then the image of the next line is recorded at one time. This process is repeated to record the entire image on the recording medium.
The ink jet recording apparatus records the image by discharging ink to the recording medium from the recording means (a recording head). It can reduce a size of the recording means, can record a fine image at a high speed, can record without requiring special processing to a plain paper, is inexpensive in a running cost, is of low noise because this of a non-impact type, and can easily record a color image by using multi-color inks.
The ink jet .type recording means (recording head) for discharging ink by utilizing a thermal energy can be readily manufactured to have a high density liquid path arrangement (discharge port arrangement) by forming an electro-thermal converter, electrodes, liquid path walls and a ceiling on a substrate by a semiconductor manufacturing process including etching, vapor deposition and sputtering so that further compaction of the apparatus is attained.
In the above recording apparatus, when the recording medium such as paper sheet or plastic sheet is to be transported (fed) in front of the recording means (recording head), it is necessary to present an error in the record position and the deformation of the recording medium. In the ink jet recording apparatus, it is necessary to prevent the contact of the recording head to the recording medium. To this end, a platen for supporting the recording medium is arranged to oppose to the recording head and a pair of rollers (usually a sheet ejection roller pair) having a transport speed equal to or slightly higher than that of the main transport unit are arranged downstream of the recording head as viewed in the transport direction in order to eliminate the relaxation of the recording medium.
As for the platen, it is necessary to maintain high precision parallelism between the platen (which supports the recording medium) and the outer peripheries of the transport roller (sheet feed rollers) in order to prevent the twist of the recording medium when it is fed. In the ink jet recording apparatus, in order to prevent the recording medium from contacting to the recording head and maintain the recording precision, it is required to keep a gap between the recording medium and the recording head (discharge port formation plane) (hereinafter referred to as a sheet gap) at minimum and constant. Thus, in the platen for-supporting the recording medium to keep the sheet gap constant, it is required to strictly control the position precision of mounting, planarity and warp. As a result, severe precision of the platen itself is required and the strict control to the platen assembly process is required.
In the prior art recording apparatus, however, since the platen is mounted on a frame member which pivotably supports the transport rollers, the assembly precision of the platen is significantly affected by the precision of the frame member.